Robot ankle is better than springs

Appropriately powered by a motor, the exoskeleton reduced the energy cost of running by 15% compared with running without the exoskeleton, according to Stanford University, where the research took place. Whereas if the exoskeleton was powered to mimic a spring, running was 11% harder than running exoskeleton-free.

“When people run, their legs behave a lot like a spring, so we were very surprised that spring-like assistance was not effective,” said Stanford mechanical engineer Steve Collins. “We all have an intuition about how we run or walk but even scientists are still discovering how the human body allows us to move efficiently.”

Fitted without power, the exoskeleton made it 13% harder to run than with nothing fitted, so ‘spring’ assistance was only 2% easier than the non-powered exoskeleton.

There was one exoskeleton per leg, operated by remote motors through Bowden cables.

Each was single-acting, pulling in parallel with Achilles tendon, with a pivoted (at the ankle) carbon fibre frame around the shin and foot transferring this force to a downwards force under the ball of the foot, without the foot or ankle joint needing to be involved.

In spring-like mode, the system mimicked a spring running parallel to the calf, storing energy during the beginning of the step and unloading that energy as the toes push off.

In powered mode, it helped extend the ankle at the end of a running step.

“Powered assistance took off a lot of the energy burden of the calf muscles. It was very springy and very bouncy compared to normal running,” said researcher Delaney Miller. “Speaking from experience, that feels really good. When the device is providing that assistance, you feel like you could run forever.”

Eleven runners tested the exoskeleton ankles, with each acclimatised to using them. In each case, exoskeleton response was tuned to the individual’s gait cycle and phases. Human energetic output assessed by measuring breathing oxygen and carbon dioxide levels.

According to the university, energy savings indicate that a powered exoskeleton could boost running speed by as much as 10%.

“You can almost think of it as a mode of transportation,” said mechanical engineer Guan Rong Tan. “You could get off a bus, slap on an exoskeleton, and cover the last one-to-two miles to work in five minutes without breaking a sweat.”

Stanford University worked with Carnegie Mellon University, Ghent University and Nike.

The work is covered in ‘Improving the energy economy of human running with powered and unpowered ankle exoskeleton assistance‘ published by Science Robotics.

Image credit: Farrin Abbott